Third order, pressure gradient responsive microphone



y 21, 1957 v. A.'SCHLENKER 2,793,255

THIRD ORDER, PRESSURE GRADIENT RESPONSIVE MICROPHONE Filed NOV. 13, 1950 m mmz VESPER A. 'SEHLENKER ATTORNEY THIRD ORDER, PRESSURE GRADIENT RESPONSIVE IVIICROPHONE Vesper A. Schlenker, Medford, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application November 13, 1950, Serial No. 195,196

11 Claims. (Cl. 179-121) The present invention relates to sound translating apparatus, and more particularly to a third order, pressure gradient responsive microphone such as disclosed in the copending application of A. H. Kettler and M. E. Hawley, Serial No. 195,204, filed concurrently herewith, patented January 11, 1955, Number 2,699,473 and assigned to a common assignee.

In a third order, pressure gradient responsive microphone of the type disclosed in the above identified application, third order operation is elfected with but a single pressure-sensitive member. This is accomplished by picking up acoustic energy at four points and transmitting the energy to acoustic chambers on opposite sides of the pressure-sensitive element through tubes or conduits. For successful operation, it is required that the sound transmitting tubes be substantially symmetrical acoustically. As pointed out in the aforesaid application, this may be done by making the tubes of equal lengths and exercising care to keep the cross-sections the same throughout their lengths. Although successful third order operation may be obtained through the use of tubes, considerable difiiculty is encountered in construction to keep them substantially symmetrical acoustically so that they will provide substantially equal acoustical impedances.

One object of the present invention, therefore, is to provide an improved structure for a third order, pressure gradient responsive microphone.

Another object of the present invention is to provide an improved structure for a third order microphone which utilizes a minimum number of parts and which requires but a single pressure sensitive element.

Still another object of the present invention is to provide an improved noise-cancelling microphone of the third order, pressure gradient responsive type which requires but a single pressure-sensitive element and which is mechanically symmetrical.

A further object of the present invention is to provide an improved microphone structure for third order operation which is simple and easy to construct and assemble and which can be produced at a minimum of cost.

The third order, pressure gradient responsive microphone provided by the present invention comprises a casing having a single pressure-sensitive element mounted therein and arranged with separate acoustical chambers on opposite sides thereof. Sound transmitting channels are provided in the walls of the casing which extend to the outer surface of the casing and connect the acoustical chambers with the ambient. The distances between the channels at the points where they open on the outer surface of the casing and the acoustical impedances of the sound transmitting channels are controlled to bear ratios to each other in a manner to provide third order operation.

The novel features characteristic of the present invention, as well as additional objects and advantages thereof, will be better understood from the following detailed description when read in connection with the accompanying drawing in which,

nited States Patent Patented May 21, 1957 Figure 1 is a plan view of a preferred embodiment of a third order, pressure gradient responsive microphone, in accordance with the present invention,

Figure 2 is an end view of the microphone shown in Figure 1,

Figure 3 is an exploded, perspective view of a portion of the microphone shown in Figure 1, and

Figure 4 is a wiring diagram of a simplified electrical circuit equivalent to the acoustical network of the vibrating system of the microphone shown in Figure 1.

Before referring to the accompanying drawing in greater detail, it is desired to point out the background theory outlined in the aforementioned copending application which prompted the development of the present invention. In accordance with this theory, if it is desired to build a third order, pressure gradient responsive microphone, instead of using eight pressure microphones, only five are necessary since one can utilize pressures P2, P3 and P4 twice. Thus, for the first pressure diflierence one obtains (P1-P2), for the second pressure difference one has (Pa-P3), for the third pressure difference one has (P3 P4) and for the fourth pressure difference (Pt-P5). These may be combined and expressed as follows to obtain third order operation:

It will be noted that the third pressure P3 cancels out. Therefore, it is not necessary to utilize the microphone providing pressure P3 at all. Thus, a third order, pressure gradient responsive microphone may be constructed using a single pressure-sensitive element. According to Expression 1 above, five pressure-sensitive elements must be considered in the case of a third order microphone. These elements may be denoted as the first, second, third, fourth and fifth elements respectively. Although it is necessary to take into account all five elements, Expression 2 shows that only four elements are actually necessary. The output of the third element is not used but is taken into account in the spacing of the other four elements. In other words, a third order microphone may be provided by admitting acoustic energy to a single vibratory element or diaphragm through four apertures (denoted herein as first, second, third and fourth elements corresponding, respectively, to the pressures at the first, second, fourth and fifth positions in space referred to in Equation 2 above), directing the energy to one side of the diaphragm through the first and third apertures and to the other side of the diaphragm through the second and fourth apertures and making the diaphragm movement twice as sensitive to a unit quantity of energy entering either the second or third apertures as to a unit quantity of energy entering either the first or fourth apertures.

In accordance therewith, a third order microphone, illustrated in Figures 1 and 2 is provided which comprises a housing or casing 1 within which a single diaphragm or pressure-sensitive member 3 is mounted in a manner to respond to sound pressure impinging on opposite surfaces thereof. The housing 1 comprises a plurality of inner and outer discs 5, 7 which have radially disposed grooves or slots 9 provided therein. The discs 5, 7 are concentrically arranged in stack formation and with the radial grooves 9 disposed such that they open at spaced intervals on the cylindrical outer surface of the housing 1, as particularly shown in Figure 2. Four such openings, 11, 13, 15, 17 are provided which correspond, respectively, to the first, second, third and fourth elements referred to above. The openings 11, 13, 15, 17 are arranged in series relation, two of the openings 11, 17 occupying extreme positions and the other two openings 13, 15 occupying means positions. As shown particularly in Figure 2 of the drawing, the openings are also arranged alternately on opposite sides of the plane including the pressure-sensitive member 3 so that successive openings in the series are connected to alternate acoustic chambers. Each of the radial slots 9 is made of uniform cross section throughout so as to provide tubes of uniform cross section and equal imped-ance.

In order that the second and third openings will provide twice the sensitivity to the diaphragm that the first and fourth openings provide, they are made twice as large. In other words, the second and third openings are dimensioned to provide smaller acoustical impedances than the first and fourth openings. This is accomplished by pairing a single radial slot 9 in each of the outer discs 7 with one of the two radial slots 9 in the inner disc 5 adjacent thereto. A thin separator 19 is disposed between the inner and outer discs 5, 7 in order to keep the acoustical impedances of the slots 51 forming the second and third apertures substantially equal. The diaphragm 3 is mounted between the inner discs 5 and is spaced therefrom at the peripheral edge by thin, annular spacing members or washers 21 so as to provide acoustical chambers 23 on opposite sides of the diaphragm. Each of the radial slots 9 is connected at the center of the discs 5, 7 with a cylindrical sound passage 25 provided by circular cut-away portions. The sound passages 25 of the outer discs 7 are not cut through the entire thickness of the outer discs so that a closed passage is provided to the acoustical chambers 23.

An electro-mechanical converter 27 is mounted on the exterior of the housing 1 by suitable attaching means and is connected to the diaphragm 3 by a drive rod 29. The drive-rod 29 extends through an opening 31 in one of the outer discs 7. A gasket 33 of rubber or other suitable material is disposed around the drive rod in the disc opening 31 to acoustically seal the opening. One end of the drive rod 29 is attached to the center of the diaphragm 3 and the other end is attached to an armature 35 of the electro-mechanical converter 27. Upon movement of the diaphragm 3 in response to a difference in sound pressure on opposite sides thereof, the armature 35 will vibrate directly with the diaphragm and produce an electrical signal, in a manner well known in the art.

The acoustical circuit for such a third order microphone may be illustrated by a simplified electrical circuit, as shown in Figure 4, wherein:

R1=the lumped acoustical resistance of the first opening of pressure-sensitive point;

R2zth lumped acoustical resistance of the second opening or pressure-sensitive point;

R3=the lumped acoustical resistance of the third opening or pressure-sensitive point;

R4=the lumped acoustical resistance of the fourthopening or pressure-sensitive point;

Rn=the lumped acoustical resistance of the diaphragm;

Li the lumped acoustical inertance of the first opening or pressure-sensitive point;

L2=the lumped acoustical inertance of the second opening or pressure-sensitive point;

L3=the lumped acoustical inertance of the third opening or pressure-sensitive point;

L4==the lumped acoustical inertance of the fourth opening or pressure-sensitive point;

I Ln=the lumped acoustical inertance of the diaphragm;

C1=the lumped acoustical compliance of the cavity on one side of the diaphragm;

C2=the lumped acoustical compliance of the cavity on the other side of the diaphragm;

Cn=the lumped acoustical compliance of the diaphragm;

V1=the sound pressure at the first opening or pressuresensitive point leading to one surface of the diaphragm;

Vz=the sound pressure at the second opening or pressure-sensitive point leading to the other surface of the diaphragm;

V3=the sound pressure at the third opening or pressure-sensitive point leading to the first mentioned surface of the diaphragm;

V4=the sound pressure at the fourth opening or pressure-sensitive point leading to the second mentioned surface of the diaphragm;

i =the acoustical volume current through the diaphragm.

The conditions upon the sensitivity of the diaphragm may then be stated as follows: (1) if the sound pressure at all the apertures is identical at the same time, the voltage out-put will be zero; and (2) it sound pressure p (where p=any arbitrary sound pressure) is incident upon the second aperture and the pressure at all the other apertures is zero, the displacement of the diaphragm will be twice that obtaining when the same sound pressure p is incident upon the fourth aperture and the pressure at all the other apertures is zero. The same analogy may be drawn with respect to apertures one and three. In terms of the electrical circuit shown in Figure 4, that is, (1) when V1=Vz =V3:(); (2) when V1=V3=V4 O and V2==v (where v=any arbitrary voltage), the potential difference across C2 will be twice the value obtaining when Vl=V2=V3:-TO and Vr v; (3) but when V1: V2=V4.=0 and V3=v, the potential difference across Ci will be twice the value obtaining when Vz=V3=Vt=O and V1=v.

These conditions may be satisfied in different ways. For example, in the particular embodiment illustrated, the first condition is met by letting C1=C2, and R1=R4, R3=Rz, Ll=L 1, L3;Lz. The second condition is then met by selecting the ratios R1/R3 and Lil/L3 so that i is twice as large when V1=V2;V4:0 and V3=v, as it is when V2=Vs=V4=O and Vi=v. Under these conditions, it will be found that L1 should equal 2L3 and R1 should equal 2R3.

As pointed out in the aforesaid copending application, it is not necessary to keep the distance between the successive points at which the pressure is observed constant in order to obtain efficient operation. However, if these distances are not the same the calculation of the proper impedance relationships becomes more difficult. The distances between the apertures should also be as large as possible in order to obtain high sensitivity. The limit on this distance is the effective phase differences between the points at which the pressure is observed when the distances between those points become an appreciable part of the wavelength. Therefore, insofar as both sensitivity and fidelity are concerned, it is generally most efficient to maintain the distances between the points at which the pressure is observed a constant. For example, in the case of the third order microphone illustrated and described herein, each of the openings occupying an extreme position is spaced apart from its next adjacent mean opening a distance equal to one-half the distance between the openings occupying mean positions.

From the foregoing description, it will be apparent that the present invention provides an improved structure for a third order, pressure gradient responsive microphone utilizing a single, pressure-sensitive element. The use of stacked discs or plates facilitates assembly and enables better control over the acoustical impedances of the sound transmitting channels since the channels can be more accurately and uniformly dimensioned by a relatively simple machine operation such, for example, as milling the slots in the discs to a fixed depth.

Although only a single, preferred embodiment of the present invention has been illustrated and described herein, it should be obvious to persons skilled in the art that various changes and modifications are possible within the spirit of the invention. For example, the openings on the outer surface of the microphone housing need not be disposed alternately on opposite sides of a plane including the diaphragm but may be arranged in alignment in that plane or in other manner of orientation which will satisfy the requirements for third order operation. Therefore, it is desired that the particular form of the present invention described herein shall be considered as illustrative and not as limiting.

What is claimed is:

1. A third order, pressure gradient responsive microphone comprising a housing, pressure-sensitive means mounted within said housing, means providing separate acoustical chambers on opposite sides of said pressuresensitive means, means connected with said pressuresensitive means for converting vibrations thereof into corresponding electrical signals, said housing in the exposed surface thereof being provided with at least four openings disposed in series, spaced apart relation and connecting said acoustical chambers with the ambient, two of said openings occupying extreme positions and two of said openings occupying mean positions, said extreme openings being so dimensioned as to provide a larger acoustical impedance than said mean openings, and means connecting successive ones of said series of openings with different ones of said chambers.

2. A microphone as defined in claim 1 wherein each of said extreme openings has an acoustical impedance which is equal to twice that of either of said mean openrngs.

3. A microphone as defined in claim 1 wherein each of said extreme openings is spaced apart from its adjacent mean opening a distance equal to one-half the distance between said mean openings.

4. A microphone defined in claim 1 wherein said pressure-sensitive means comprises a single pressure-sensitive element.

5. A microphone as defined in claim 1 wherein said pressure-sensitive means comprises a single vibratory member and wherein each of said extreme openings has an acoustical impedance which is equal to twice that of either of said mean openings, and wherein each of said extreme openings is spaced from its adjacent mean opening a distance equal to one-half the distance between said mean openings.

6. A third order, pressure gradient responsive microphone comprising a housing, a single pressure-sensitive element mounted within said housing, means providing separate acoustical chambers on opposite sides of said pressure-sensitive element, and means connected with said pressure-sensitive element for converting vibrations thereof into corresponding electrical signals, said housing in the exterior surface thereof being provided with two pairs of openings, and means connecting said chambers with the ambient through said openings, said openings being disposed in series in spaced apart relation alternately on opposite sides of the plane including said pressuresensitive element to provide a pair of extreme openings and a pair of mean openings, said extreme openings each being dimensioned to provide a larger acoustical impedance than either of said mean openings, successive ones of said series of openings being connected to alternate ones of said chambers.

7. A microphone as defined in claim 6 wherein said mean openings are spaced apart a distance equivalent to twice the distance between either extreme opening and the next adjacent mean opening.

8. A microphone as defined in claim 6 wherein each of said extreme openings has an acoustical impedance equal to twice the acoustical impedance of either said mean opening.

9. A microphone as defined in claim 6 wherein said mean openings have equal acoustical irnpedances, said extreme openings have equal acoustical impedances, and wherein each said extreme opening has an acoustical impedance equal to twice the acoustical impedance of either of said mean openings.

10. A microphone as defined in claim 6 wherein said acoustical chambers have equal acoustical impedances.

11. A third order, pressure gradient responsive microphone comprising a housing provided by a pair of inner discs and a pair of outer discs, said discs being arranged in stack formation, a single pressure-sensitive element mounted between said inner discs, means providing separate acoustical chambers on opposite sides of said pressure-sensitive element, said discs having radially disposed slots therein extending outwardly to the periphery thereof to provide four openings on the exterior surface of said housing, and means connecting said slots with said acoustical chambers, said openings being arranged in series spaced apart relation, successive ones of said series being connected with alternate ones of said chambers, the first and last of said series of openings each being dimensioned to provide a larger acoustical impedance than either the second or third opening of said series.

References Cited in the file of this patent UNITED STATES PATENTS 2,301,744 Olson Nov. 10, 1942 2,305,599 Bauer Dec. 22, 1942 2,396,222 Foldy Mar. 5, 1946 2,529,467 Wiggins Nov.7, 1950 2,552,878 Wiggins May 15, 1951 

